3.1.2 Carbohydrates

Question 1

what are monosaccharides?

Answer 1

The individual sugar monomers from which larger carbohydrates are
made.

Question 2

what are disaccharides?

Answer 2

formed by the condensation of two monosaccharides

Question 3

what are 3 common types of monosaccharides and examples?

Answer 3

triose - glyceraldenyde
pentose - deoxyribose
hexose - glucose, fructose, galactose

Question 4

what is an isomer?

Answer 4

same molecular formula but different structural formula (arrangement of atoms)

Question 5

what is the difference between alpha glucose and beta glucose?

Answer 5

in alpha glucose, the hydroxyl group in carbon atom 1 is below the hydrogen whereas in beta glucose the hydroxyl group in carbon atom 1 is above hydrogen

Question 6

what is a glycosidic bond?

Answer 6

A bond between two monosaccharides formed in a condensation reaction.

Question 7

how is maltose formed?

Answer 7

condensation of two glucose molecules

Question 8

how is sucrose formed?

Answer 8

condensation of a glucose molecule and a fructose molecule

Question 9

how is lactose formed?

Answer 9

condensation of a glucose molecule and a galactose molecule

Question 10

what is the process of the benedicts test?

Answer 10

• add 2cm^3 of test solution
• add an equal amount of benedict’s solution
• heat in a water bath

Question 11

what is a positive test of benedict’s solution?

Answer 11

brick red precipitate

Question 12

what is a negative test of benedict’s solution?

Answer 12

blue

Question 13

how do you know the concentration of a reducing sugar?

Answer 13

• high conc. = further colour change e.g brick red
• compare to different solutions, or more accurately, filter solution and weigh precipitate
• or remove precipitate and use a colorimeter to measure absorbance of remaining benedict’s reagent

Question 14

how do you test for non-reducing sugars?

Answer 14

• add dilute HCl to the test solution + carefully heat in a water bath
• you then neutralise it by adding sodium hydrogencarbonate
• retest the mixture using the test for reducing sugars (Benedict’s solution) - the non-reducing sugar will have been hydrolysed into monosaccharides which are reducing sugars

Question 15

what is an anomeric carbon?

Answer 15

carbon of the carbonyl group present in the straight chain of the sugar

Question 16

why do non-reducing sugars not reduce benedict’s solution?

Answer 16

sugars reduce benedict’s solution when the anomeric carbon atom is made available to reduce the copper ions in the solution
- e.g the anomeric carbon atom of glucose is 1, for fructose is 2 - sucrose is a 1-2 glycosidic bond so the anomeric carbon atoms have been taken up

Question 17

why are reducing sugars called reducing?

Answer 17

• Cu2+ ions are reduced to Cu+ in benedicts solution - which form copper (I) oxide in the presence of alkaline sodium hydroxide

Question 18

what does polysaccharide mean?

Answer 18

many sugars

Question 19

what monosaccharides are polysaccharides made of?

Answer 19

alpha-glucose OR beta-glucose

Question 20

what’s the purpose of starch?

Answer 20

plants store excess glucose as starch (starch can be hydrolysed back into glucose when plants require glucose for energy via respiration)
- mixture of two polysaccharides - amylose and amylopectin - make up starch
- it is also insolutuble - no osmotic effect (doesnt cause water to enter cell by osmosic which could make cell swell, this is good for storage)
- it is a large molecule - cannot diffuse out of cells bcause it can’t pass the cell membrane

Question 21

what is amylose?

Answer 21

• alpha-glucose, 1-4 glycosidic bonds
• long, unbranched chain
• coiled structure - compact, stores lots of glucose

Question 22

what is amylopectin?

Answer 22

• alpha-glucose
• 1-4 and 1-6 glycosidic bonds meaning it’s branched - allows enzymes to hydrolyse the glycosidic bonds easily to rapidly release glucose for respiration which released energy quicker

Question 23

what is glycogen?

Answer 23

• store of excess glucose in animals
• alpha-glucose
• 1-4 and 1-6 glycosidic bonds
• coiled - compact - good for storage
• more highly branched than starch (allows enzymes to hydrolyse the glycosidic bonds easily to rapidly release glucose for respiration which released energy quicker)
• insoluble - no osmotic effect
• large molecule - cannot diffuse out of cells bcause it can’t pass the cell membrane

Question 24

what is cellulose?

Answer 24

• found in plants (make up cell wall)
• insoluble - no osmotic effect
• beta-glucose, 1-4 glycosidic bonds - hydroxyl groups on c1 and c4 are too far to react, so every other beta-glucose is inverted 180º (upside down).
• form long, straight, unbranched cellulose chains
• cellulose chains linked by many hydrogen bonds (weak by itself, strong + rigid when there’s many)
• cellulose chains join to form microfibrils which join to form macrofibrils which combine to make strong cellulose fibres in the cell wall of plants
• provide structural support of cells

Question 25

how do you do the iodine test for starch?

Answer 25

• add 2cm^3 of sample
• add few drops of iodine in potassium iodide

Question 26

what is a positive test of the iodine test?

Answer 26

blue-black

Question 27

what is a negative test of the iodine test?

Answer 27

orange-brown